An alien species (of name and provenance that shall remain unmentioned) have an exotic metabolism.

They are apparently equally comfortable in atmospheres of oxygen, chlorine, or fluorine, and are capable of using any of those gasses as metabolic oxidizer.

What might their biology be like if they can usefully breathe oxygen and chlorine for energy, but are simultaneously unharmed by exposure to elemental fluorine?

A list of all of the Anatomically Correct questions can be found here: Anatomically Correct Series

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    $\begingroup$ I’m voting to close this question because it's asking a question about an existing 3rd party world and not asking about building a new fictional world. Current site policy is that 3rd party worlds are off topic for this site. $\endgroup$
    – sphennings
    May 15 at 18:03
  • $\begingroup$ Kearsley your lightning rod is mentioning prior works / existing fiction. Can't have that on WB stack now it seems. I took the liberty of deleting that part of your question so it will not be closed as I mull over exotic oxidizer biochemistry. $\endgroup$
    – Willk
    May 15 at 19:33
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    $\begingroup$ How in the frilly-heck could we decide anything whatever about it's anatomy? Crocodile, giraffe and cockroach all have the same basic requirements gas-wise - yet their anatomy is vastly different, do you see the issue? It's physiology - we might figure something about that, but anatomy? Not much chance without an undefined and very broad field with no obvious best answer. Way too broad. $\endgroup$ May 15 at 21:10
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    $\begingroup$ A.E Van Vogt's The War Against the Rull @Tortliena "six-legged saurian-like creature" - 3rd part world. $\endgroup$ May 15 at 22:24
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    $\begingroup$ @LoganR.Kearsley Nay, there should be no exception, regardless of the question tag. However and unlike a recent another question, the description of the alien is too "unspecific" and don't match the "flying, chameleon worm/snake" you see here and there. However and ever, since it's so undefined, it's not an "anatomically-correct" in the SE sense. You're just asking how you can make an alien breathe multiple kinds of air. You should remove the anatomically-correct reference, that will remove any confusion! $\endgroup$
    – Tortliena
    May 15 at 23:49

3 Answers 3


Difficult to imagine due to the need for reversibility

One of the requirements of biochemistry is that there is a certain amount of reversibility. Take haemoglobin as an important example. It gets oxygenated and deoxygenated in cycles.

Anything that binds reversibly to chlorine is likely to either not bond at all, or bond completely irreversibly to fluorine, and vice versa, depending on the sterics (geometry) and Lewis hardness/softness of the binding site.

This isn't just a thing in hemoglobin, either. Many of the chemical building blocks of your body need to be disassembled and reassembled repeatedly.

If I had to hazard a guess, I'd say you'd need some Super Hemoglobin with three different active sites, one for each of F, O and Cl, and the bound species would be used for different things; F for things that are never being disassembled, like a carapace or skeleton, O for general purpose biochemistry, and Cl for, I don't know, making lots of HCl to spit at combatants, or metabolising metal for energy.


All terminally fluorinated.

I imagine a being that swims in a sea of molten salts and metals. Above is an atmosphere of harsh oxidizers as described. The sea is not full of these oxidiziers- oxidized metals form a crust at the surface and protect the liquids below.

When the creature surfaces and breathes, all internal components that come into contact with the atmosphere are terminally fluorinated - like calcium fluoride or metal fluorides. These substances cannot accomodate more fluoride and the fluoride these substances has will not be displaced by chlorine or oxygen.

This creature has a boiler type metabolism and the boiler is lined with these fluorinated molecules. The creature generates heat from oxidation of oxidizables and the heat from its boiler is used to catalyze other reactions that produce metabolic energy.

Fluorinated and other oxidized substances are waste and join the floating crust. This is periodically regenerated into the metals and oxidizer gases by electrolytic storms, with O2, F2 and CL2 gas evolved that bubbles from the surface back into the atmosphere. These electrolytic storms are not compatible with life and macroorganisms like the one in this scenario vacate the area when one is brewing.


The main barrier here is the fluorine resistance; once fluorine has bonded to something, chlorine or oxygen will not replace it! However, if the creature's tissues aren't already fluorinated, they will rapidly become so in a fluorinating atmosphere! So, if the creature's tissues must be naturally fluorinated, how could it possibly survive on anything other than fluorine?

They key is to recognize that it can't be fully fluorinated, just as we aren't fully oxidized! Whatever the details of this fluorinated biochemistry are, there must be unsaturated bonding sites which could be broken up for energy and occupied by oxygen or chlorine--otherwise, they'd just be a cloud of CFl4 or SiF4 gas or something like that, rather than an organism made up of complex biomolecules. And while fluorocarbons are famously not flammable in air, that doesn't mean they won't exothermically react with oxygen at all--just that the activation energy is too high for the reaction to be self-sustaining. Plenty of fluorocarbons will, however, combust in higher concentrations of oxygen--and overcoming activation energy is what enzymes are all about!

Fluorocarbon liquids are also famous for their ability to dissolve gasses. They are, in fact, used for liquid breathing, because they can dissolve sufficient oxygen for respiration directly. Thus, given perfluorocarbon-based blood, these creatures would not need to contend with developing reversibly-binding hemoglobin-equivalents for all three possible oxidizers as was brought up in Sean OConnor's answer.

So, for simplicity, assume the creatures are still carbon-based. Their biomolecules will have a lot of fluorine atoms substituting where we would use terminal hydrogens or -OH groups, such that they are not corroded by fluorine gas. Being resistant to fluorine, they are also unbothered by the chemical reactivity of oxygen and chlorine. Chlorine might be a metabolic poison, in the way that, e.g., carbon monoxide is a poison for us (because it displaces oxygen but is not oxygen), or arsenic (pretends to be phosphorus) or strontium (pretends to be calcium)--but we'll suppose that they have instead evolved more specific enzymes that can discriminate between chlorine and fluorine. Their body tissues, being perfused with perfluorocarbon liquids, directly dissolve large quantities of oxygen, fluorine, and chlorine from the air (along with other gasses), so all three are available to respiration, so long as they are available in the environment.

When they have access to fluorine gas, that's the preferred oxidizer, and the creatures would end up exhaling mostly CF4 and HF, with some COF2 (assuming that oxygen is still used as a frequent heteroatom in their biochemistry--and why wouldn't it be?), possibly some free oxygen (OF2 production is not thermodynamically favorable at normal pressures), and NF3 as a result of protein metabolism; no need for these guys to produce urea!

Given access to oxygen but no fluorine, they would fall back on a less-energetic system, producing almost exclusively COF2 (the result of reaction -CF2- monomers with oxygen), and probably some water. Terminal trifluormethyl groups would be unnattackable, but those could be packaged and eliminated as hexafluorethane. Protein metabolism could just release gaseous nitrogen directly--high gas solubility means that they still have no need to package waste nitrogen as urea!

Finally, chlorine metabolism would be the last resort, producing the least energy, but still more than anaerobic respiration / fermentation. Perfluorocarbon chains would produce primarily CF3Cl (Freon-13) and CF2Cl2 (Freon-12) waste gasses, along with COCl2 (phosgene)--there would be no free oxygen produced, because chlorine (unlike fluorine) won't displace oxygen, so it would just fill the extra slots of carbons that already have C-O bonds. And just like OF2, OCl2 is not thermodynamically favorable at low pressure.

Given that they can breathe fluorine, there's probably some hydrofluoric acid replacing water, and integration of organosilicons, but that doesn't change the general structure; the non-oxygenated carbon compounds produced as metabolic waste have direct silicon analogs (SiF4, SiF3Cl, SiF2Cl2), which are also conveniently gaseous. Silicon oxyfluorides and oxychlorides are more complicated, so Si-O bonds might be isolated from fluorine and chlorine metabolism to form silica instead, which can be dissolved in hydrofluoric acid for liquid elimination.


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